The present invention relates to telecommunications systems in general, and more specifically relates to linked compressor-expander circuits (LINCOMPEX) circuits implemented digitally.
Linked compressor and expander (hereafter LINCOMPEX) systems for telecommunications networks have generally been known since the early 1960's. Various specifications for present LINCOMPEX systems have been set forth. For example CCIR report #354, CCIR recommendation #455-1, British Post Office Specification #W6819B, and various articles and publications define specifications and implementing techniques for present day LINCOMPEX systems.
The presently developed LINCOMPEX systems have used analog circuitry. The technical requirements, such as given in the above publications, are sufficiently stringent to require the use of highly accurate analog circuits. For example, a control tone requirement of many specifications is that the center frequency must be 2900 Hz.+-.1 Hz with a ratio of change of 2 Hz per db. This sort of requirement of precision translates into increased costs, increased sizes, increased set-up and adjustment time, increased maintenance cost, and descreased reliability. A relatively large number of analog components are required, and they suffer because of susceptability to temperature and humidity conditions. The power consumed in such analog systems is relatively high, and the necessary operating controls are numerous. Because of the large unit size and weight, previous LINCOMPEX systems have, in general, not been suitable for inclusion within the cabinet of the transmitter unit.
Such analog systems have also suffered from calibration difficulties. As environmental conditions and as component operating characteristics vary, such analog systems require rather complicated procedures for calibrating the demodulator, at a remote end of a transmission medium, with a modulator at the transmitting end.
Such analog systems also have not been suitable for efficient transmitter usage. For example, periods between syllables of transmitted speech in previous LINCOMPEX systems have not been used for transmission. This results in overall inefficient telecommunications networks.